Within the framework of automatic or semi-automatic landings, the objective of radionavigation is to lead the aircraft into a space, aligned with the axis of the landing runway and with a descent slope of 3° (typical but programmable). To the knowledge of the Applicant, this is customarily carried out by virtue of onboard equipment, for example a multimode receiver better known by the abbreviation MMR, which receives the signals from various navigation systems such as: the instrument landing system or “ILS”, the MLS system (the acronym standing for the expression “Microwave Landing System”), the FLS system (“Flight landing system”) and the GLS (“Global landing system”) positioning system. The microwave landing system called MLS is a system for aiding instrument approach and landing, intended to provide an aircraft with its position in spherical coordinates in a reference frame tied to the landing runway, that is to say an angle of azimuth, an angle of elevation). The angles of azimuth and of elevation, being known to persons skilled in the art working in this field, will not be explained. The MLS, such as standardized by the ICAO worldwide organization, transmits signals for lateral guidance, that is to say an angle of azimuth, and for vertical guidance, that is to say an angle of elevation, by using a technique of referenced-time beating beams and a time-multiplexed signal. The use of a time-multiplexed signal allows the transmission of the signals for lateral and vertical guidance on the same radiofrequency channel without creating interference between the signals for lateral guidance and the signals for vertical guidance. The guidance signals are emitted on a frequency of around 5 Giga Hertz (GHz) by an azimuth station and an elevation station. The azimuth station is placed at the end of the runway while the elevation station is situated on the side of the runway, about 300 meters from the runway start threshold. Each station transmits a narrow beating beam, sweeping the coverage space back and forth at constant angular speed along the angular coordinate considered. An antenna and a receiver onboard the aircraft receive the beating beam a first time during the outward sweep and a second time during the return sweep. It is thus possible to determine the angle of azimuth or the angle of elevation through the following linear relation:
                    θ        =                              (                          T              -                              T                0                                      )                    ·                      v            2                                              (        1        )            where θ is the angle of azimuth or the angle of elevation,
T a time interval between the reception of the outward and return scans of the beating beam,
T0 the value of the time interval T for a zero angle θ and
ν the angular sweep rate.
T0 and ν are constants defined by the international standards for MLS, known from the prior art.
Offset approaches are approaches which are carried out using the MLS signals of the main runway (where the MLS beacons are situated), but for aiding landing on a runway parallel to this runway and covered by the signals of the beacons. This type of approach is described in standard D0226 (previously in standard DO198), standards known to the person skilled in the art in the field of radionavigation.
The algorithms proposed by standard DO226 make it necessary to obtain distance information. This distance information is obtained either via a measurement device better known by the acronym DME (abbreviation of Distance Measuring Equipment), or via GPS information (by following for example the method described in the application filed by the applicant FR 07 09035). A drawback of proceeding in such a way is notably the need to equip the aircraft with an additional device, this possibly turning out to be expensive.